I. Field of the Invention
The present invention relates generally to communications. Particularly, the present invention relates to symbol recovery from a binary data stream.
II. Description of the Related Art
A received signal comprising a symbol stream may not be time aligned with the receiver. In such a case, the receiver needs to know when to sample the received signal in order to accurately recover associated data with lowest probability of error.
Typically, a delay-locked loop is used to identify transitions which occur in the received signal. This enables sampling to occur away from such transitions.
The delay-locked loop is a general-purpose solution to the timing recovery problem that yields moderate performance. However, when the received signal contains some pre-defined sequences of transitions that are known to both transmitter and receiver, one can do better than the general-purpose delay-locked loop by utilizing a priori knowledge of the signal content. In other words, to determine the correct timing alignment, the received message can be correlated against a pre-defined sequence of data known by the receiver to be present in the received message. This technique is referred to as the xe2x80x9csync word correlationxe2x80x9d method of timing recovery, where xe2x80x9csync wordxe2x80x9d denotes the pre-defined sequence of data.
For oversampled received signals, (e.g., 1 Mbps information signal sampled at 12 MHz) there may be many oversamples that correspond to each information symbol. Thus, when the xe2x80x9csync word correlationxe2x80x9d method is applied to an oversampled signal, there may be multiple timing alignments where the sync word is detected within the received signal. This is especially so given that hard decisions have already been made on the individual oversamples, prior to the timing recovery.
For minimum probability of error, it is desired to choose the timing alignment that corresponds to the xe2x80x9ccenterxe2x80x9d of the information symbol, because this is the maximum eye opening in an eye diagram that is used to depict ISI (inter-symbol interference). Choosing a timing alignment that is not the center of the symbol may result in a higher probability of error, since in general such an alignment is closer to the edge of the symbol, and is more susceptible to noise than the alignment at the center of the symbol. However, the process of identifying which alignment out of the multiple possible timing alignments corresponds to the center alignment is not obvious. Hence, there is a need for a robust and simple way of determining the center timing alignment.
The present invention encompasses a process for recovering symbols from a binary data stream that is received at a predetermined data rate. The binary data stream is oversampled at a predetermined oversample rate to generate an oversampled signal. In one embodiment, the data rate is 1 MHz and the oversampling rate is 12 MHz. The incoming received oversampled signal is correlated with a pre-defined sequence known to both receiver and transmitter, called the xe2x80x9csync word.xe2x80x9d The output of the correlation is then convolved with a linear filter such as a moving average window of variable width. The output of the convolution is the convolution signal that is then scanned for a peak. The alignment corresponding to the peak is identified as the xe2x80x9ccenterxe2x80x9d of each symbol in the received signal.